(1) Field of the Invention
The invention relates to antennas and is directed more particularly to a design for compact high frequency transmitting antenna capable of over the horizon communication.
(2) Description of the Prior Art
Beyond line of sight or over the horizon communication is possible using the satellite, the aircraft relay, ionospheric (sky-wave) propagation and ground-wave propagation (i.e. AM broadcast stations). Applications for which the use of a satellite or airborne relay is not feasible (due to availability or cost) are left only with either ground-wave or ionospheric HF-band propagation. Ionospheric propagation occurs most effectively at frequencies between 2 MHz and 30 MHz, known as the HF or shortwave band. The use of either of these HF-band propagation modes is not new and actually dates back to the beginning of radio.
Antennas, regardless of frequency, generally must be electrically large (at least ¼ wavelength at the operating frequency) in order to be effective radiators. At the HF-band frequencies, particularly those between 2 and 30 MHz, the wavelengths involved (150 meters and 10 meters, respectively) require using antennas that are physically too large for small systems such as a personal radio or a compact (<0.5 cubic foot) at-sea buoy. Although matching networks can be used, these have a number of drawbacks including poor frequency flexibility, narrow bandwidth, poor efficiency, and a tendency to suffer environmental performance degradation or “detuning”. Environmental performance degradation is caused by insufficient height above a conducting surface and surrounding objects.
Radio systems incorporating such match networked antennas can still be effective, if sufficient transmitter power is available to overcome the antenna inefficiency, the frequency of operation is fixed and the location is held constant. These parameters cannot exist in many applications such as for a buoy that is small, has low battery capacity, needs frequency flexibility, and is located in a highly variable environment.
The main reason that these drawbacks exist for electrically small antennas is that traditional radio transmitting systems utilize a radio frequency power source or transmitter that is designed to operate with a load close to 50 ohms. This load is a world-wide industry standard for interconnection of radio equipment. Unfortunately, an electrically small antenna typically presents a much higher load (perhaps by a factor of 100, based upon size) than 50 ohms. This load is typically connected to a radio through a standard 50-ohm coaxial transmission line. The antenna must be provided with a matching network to transform its natural load impedance (perhaps 2000 ohms in magnitude, mostly capacitive reactance) to 50 ohms. Without use of a matching network, enormous losses are encountered within the transmission line. Such a matching network requires space and its performance suffers with changes in environment. Efficient matching networks are much less adaptable than networks having significant losses. Losses are often deliberately included within many matching networks for improving unattended performance.
Ultra-wide band radio technology is vastly different from classical radio transmission. Extremely short pulses are generated at the baseband frequency and are transmitted without the use of a carrier. Ultra-wide band uses extremely short pulses of electromagnetic energy that translate to very wide bandwidths in the frequency domain. In the past ultra-wide band was referred to as baseband, carrier-free and impulse technology. The FCC defines ultra-wide band as a signal with either a fractional bandwidth of 20% of the center frequency or 500 MHz. Due to the low duty cycle of the pulse the energy requirements are significantly reduced. Due to the low power spectral density the system has a low probability of intercept and low probability of detection. The wide bandwidth and low power makes ultra-wide band transmissions appear as background noise. The power spectral density (PSD) is defined as:
                    PSD        =                  B          P                                    (        1        )            Where P is the power transmitted in watts (W), B is the bandwidth of the signal in hertz (Hz), and the unit of PSD is watts/hertz (W/Hz).
The key benefits for ultra-wide band are high data rates, low equipment cost, low power requirements, multipath immunity and ranging. The large bandwidths of ultra-wide band means extremely high data rates can be achieved. Because ultra-wide band pulses are extremely short they can be distinguished from multipath because of the fine time resolution. ultra-wide band low frequency components enable the signals to propagate effectively through materials such as foliage and structures. ultra-wide band transmitters and receivers do not require components such as modulators, demodulators and IF stages. This reduces cost, size, weight and power consumption of ultra-wide band systems compared with conventional narrow-band communication systems. The low power and inexpensive components make ultra-wide band of interest for distributed sensor network systems. Since the sensors themselves require power, typically the power for the communications is only a small portion of the overall system power requirements. In addition, because of the great numbers of sensors in a sensor field and the fact that they may be expendable, system cost is a great concern. It is expected that sensor systems may not have high data rate requirements as the messages will be short. However, higher data rates allows for quicker message offload and could allow for future growth.
Impulse generation is the fundamental approach for generating ultra-wide band waveforms. Looking back in history, ultra-wide band actually had its origins in the spark gap transmission design of Marconi and Hertz in the late 1890s. The pulse characteristic (Gaussian, Gaussian monocycle, polycycle in a Gaussian envelope, Scholtz Monocycle, square pulse, etc.) can be varied to optimize the frequency spectral mask. FIG. 1 shows a diagram of an ultra-wide band waveform. The frequency bandwidth is on the order of the inverse of the pulse duration (τ)
                              1          τ                =        BW                            (        2        )            
The pulse can be modified by a higher speed chipping signal. This pulse rate determines the center frequency. The frequency is the inverse of the time period (T). The pulse repetition rate, R, causes spectral lines that could cause severe interference to existing narrowband radios. The data rate is directly related to the pulse repetition rate. The pulse shape determines the characteristics of how the energy occupies the frequency domain. For example, Gaussian waves provides a smoother spectral mask. However, a square pulse can easily be generated by switching a transistor on and off quickly. The most common modulation schemes for ultra-wide band systems are pulse amplitude modulation (PAM), on-off keying (OOK), Binary Phase Shift Keying (BPSK) and pulse position modulation (PPM).
There is a need for a compact antenna that can be placed on a buoy. This type of emplacement requires high efficiency because of the power limitations of the isolated buoy. Providing the greatest radio frequency voltage at the feed-point of the antenna and maintaining this voltage at as high a level as possible over as wide a range of frequencies and sea conditions as possible is also desirable. The transmitter should also have durable parts giving the greatest voltage at the lowest battery power consumption. Little work has been done in optimizing an HF-band (2-30 MHz) antenna and transmitter system for a small, expendable device or platform.